text:
Is it possible to control an aircraft on the runway if the nose gear collapses?
I have read and seen some articles in which, for various reasons, the aircraft's nose gear collapsed after landing during the roll out. This resulted in the aircraft going off the runway.
Is it possible to still control the aircraft during this weird event? Or is it more or less inevitable that the aircraft will run off the runway?

Based on the fact that you've read several articles indicating that an aircraft had the nose gear collapse during landing, then had a runway excursion, I would say "yes" there is a relationship. It might even be a causal relationship.Since you seem to have answered your own question when you asked it, what more are you looking for?

The question is: Why did this situation happen?

"Why did the gear collapse?", "Why was there a runway excursion?", or "Did one lead to the other and if so why?" Which situation, are you referring to? I'd suggest editing your question to help clarify that so you can get a good answer.

Most aircraft are steered on the ground by pivoting the nose gear. If the nose gear collapses, you therefore lose your primary method of directional control on the ground.
Additionally, parts of the aircraft contacting the ground that are not supposed to contact the ground can cause unwanted yaw, which can contribute greatly to the runway excursions you've noticed in articles regarding gear collapse incidents. This is probably the largest contributing factor to runway excursions in cases of nose gear collapse. However, if it's known that the nose gear did not deploy properly before the landing, the pilots can intentionally hold the nose gear off the ground as long as possible in order to minimize this effect. Holding the nose gear off the ground as long as possible minimizes the time that the contact between the nose and runway surface will be potentially applying yawing forces to the aircraft and also minimizes the speed of the aircraft while those forces are being applied. This gives the pilots more time to respond with differential braking and/or rudder, helping to prevent the nose from moving too far away from the direction of travel and a resulting loss of directional control due to skidding. Of course, this doesn't help if the gear collapse was unexpected, as the pilots wouldn't know to hold the nose gear up.
There are a few additional methods of directional control available to airplanes on the ground, though most are not as effective as pivoting the nose gear and their effectiveness varies with speed:

Rudder - While the aircraft still has high airspeed when rolling down the runway, the rudder will still provide some directional control. This control will diminish very rapidly as the aircraft slows down, though.
Asymmetric braking - Most airplanes allow the pilots to control the brakes on the left and right main gear independently, allowing them to brake harder on one side of the aircraft than on the other. This is more useful for taxi speeds than for high speeds down a runway, though, and can easily cause even more directional control problems if used without proper care at high speeds.
Asymmetric thrust - Multi-engine aircraft almost always allow the pilots to control the thrust (either forward or reverse) of each of the engines independently. However, in the case of a collapsed gear, this is unlikely to be very helpful, due to the large delay between when a thrust change is commanded and when it happens as well as that the engines may now also be striking the ground themselves.

Actually, frictional force doesn't increase linearly with speed.  In a simplified theoretical model of dynamic (sliding) friction, the force = coefficient * contact_force.  The energy/sec, or braking power (which turns into heat) of the friction is thus linearly proportional to speed, not quadratically.  (energy = force  distance, so the same force covering more distance per second means higher power.)  Holding the nose up as long as possible helps because you're closer to stopping when it drops and you start to yaw.  Also less frictional heating.

@PeterCordes: Also, distance traveled in any direction is proportional to velocity; so the same 5 degrees of yaw will get you off the runway three times faster at speed three times higher - leaving you a third of the time to correct with rudder and brakes.

@SF. Yup, that's what I was getting at with the last sentence about being "closer to stopping".

@PeterCordes Yeah, you're right. Fixed. Granted, yawing moment will still be somewhat related to speed because the moment arm will grow faster at higher speeds, but you're right that the force itself is (more or less) constant.

@SF. 5 degrees of yaw doesn't necessarily mean the aircraft is traveling 5 degrees off the runway center line. It means the nose is pointed 5 degrees off center line. Where the problem comes in is that the yawing moment increases because now the force is being applied even further off the track of the CG (i.e. moment arm increases.)

There is nothing wrong with all answers here, but one aspect is missing.
If the nose gear collapses, the instinctive reaction of most pilots would be to brake as much as possible. Forget reverse thrust or spoilers - the pilot will hit the brakes. If pressure builds up slightly differently between left and right brake, or if the wheel load between both main landing gears is different due to cross wind, the aircraft will experience asymmetric braking, and now the nose will not be exactly centered between both main wheels.
Now we need to look at the braking forces of the skidding front fuselage in comparison to those of the main wheels. Since the aircraft has now a pronounced nose-down attitude, its load on the nose will be quite a bit higher than the regular load on the nose landing gear used to be. With the increased load on the front fuselage, its scratching along the concrete will contribute a lot to the braking force, and if this force is off-center it will produce a yawing moment that can be balanced only initially and with full application of differential braking. Once the front fuselage has sverved beyond the tracks of the main gear, no braking will bring it back into the center and the aircraft is uncontrollable. Add some crosswind and it will leave the runway quickly.

Is it possible to still control the aircraft during this weird event?

Applying rudder will still help at high speed, but once the aircraft slows down and the rudder becomes less effective, the aircraft is uncontrollable.

Or is it more or less inevitable that the aircraft will run off the
  runway?

Only in perfectly symmetrical conditions will it stay on the runway. Since the aircraft with the collapsed nose gear is unstable in yaw, the far more likely outcome is a runway excursion.

To an extent, yes.
Most importantly you can also control the airplane using differential braking of the rear wheels. This is how planes with a free castering nose wheel are steered on the ground.
On a side note if the plane is moving fast enough and their is sufficient airflow over the rudder you may be afforded limited control of the aircraft via the rudder. However as the speed decreases (your nose is presumably dragging on the ground at this point) your rudder will lose authority and differential braking would be the way to go.

You may indeed have differential 'breaking' of the main gear in these situations (i.e. one of the main gear breaks off before the other,) but this is usually unhelpful for directional control. :)

Edit accepted, silly mistake!

@Dave: Did my edit lose your link? I don't know how that happened.

Not sure why it did that but I put it back in, still had the tab open.

Maybe you edited in the link within the grace period, but while my edit was pending.

Differential braking would still be available for directional control, and the rudder would still be effective at the same speeds as normal (effectiveness decreasing as speed decreases).  To what extent one would have the presence of mind to use differential braking after the perhaps unexpected collapse of the nose gear is a question.  If the engines (assuming a multi-engine aircraft) weren't affected by the gear collapse, differential power would also be an option, although that's not always something that can be fine-tuned, and again would require the presence of mind to use it.
I believe there was a Jet Blue aircraft that landed in LAX a few years ago with a nosegear that, while not collapsed, was useless by virtue of being cocked sideways.  My recollection is that they stopped fairly well centered on the runway, although they had the advantage of knowing the situation in advance and working out a plan for exactly what they had.  So one might conclude that an operative nosewheel steering isn't essential to staying on the runway, but there's not much doubt that it certainly helps.

Yeah, there have actually been at least a few incidents like that JetBlue flight, IIRC. AFAIK, they've all stayed on the runway, which was aided greatly by the fact that they knew they needed to keep the nose off the runway as long as possible and that the gear usually didn't actually collapse, so they weren't getting crazy yaw forces from parts of the plane touching the runway that aren't supposed to touch the runway.

In case of that JetBlue flight mentioned, they e.g. burnt off the fuel in the tanks forward of the center of gravity and moved the passengers into the tail section before landing, and possibly a couple of other things, to eek out a few seconds extra without the nose gear on the ground.


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How is performance downgraded if the windshield is damaged in flight?
A cracked windshield in flight will decrease the performance. To what extent can it disrupt the flight? Can flakes of glass be ingested on tail-mounted engines? Is it possible to blow out the windshield because of the high speed?

The biggest restriction will be the reduced flight speed and cabin pressure which need to be observed with a damaged windshield. No pressurization will restrict the flight altitude to 10,000 ft or less. The crack will not increase drag, but force the pilot to continue flight at a less efficient speed and altitude.
Why high altitude is good for engine efficiency can be found here.
How to find the most efficient flight speed can be read here.
If pieces break off, the aircraft should not continue the flight, but land immediately.
To answer the other questions: Yes, sure, tail-mounted engines are known for ingesting stuff which has broken off from the fuselage. Ice from a leaking lavatory has caused engine damage and the imbalance made the engine shake so hard that it fell off. The whole windshield or substantial fractions of it could have this effect, but I doubt that flakes will damage the engine.
The aerodynamic pressure at the windshields tends to press them into the fuselage. Suction only starts above of the windshield with the curvature of the fuselage contour. However, with cabin pressurization added, the windshield is indeed forced out if it fails. One accident which was caused by using the wrong screws to secure the windshield is BA Flight 5390. The captain was sucked out of the opening left by the window and only his ankles, which got stuck in the flight controls, prevented him from falling out entirely.

This depends on how the windshield is damaged.  Aircraft windscreens are often multi-layer and can withstand cracks in the outermost layer.  If this is the only damage then the flight can operate as normal and write up the airplane so that maintenance looks at it before returning the airplane to service.  If the inner layers are cracked then the things mentioned in Peter's answer come into effect.
Specifically in the EMB-145 I operated:

if only the outer layer is cracked, no action is necessary
if the inner layer is cracked

descend to 10,000 or MEA and maintain pressurization at $\Delta$P at 1 PSI
If both windshields are impaired

depressurize
max airspeed 140 KIAS.

In the case the inner layer is cracked, this will likely turn into a diversion coordinated with maintenance to either swap an airplane or wait for repairs to be made.  With both windshields cracked and completely obscured landing becomes a lot more interesting.

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Altimeter disagreement. What's the crew procedure to deal with that?
The crew is facing a situation, in which, the altimeters indicate different altitude (the port and starboard ones - cpt´s and FO´s). Is there any procedure to perform an IFR landing?
If we consider a light aircraft, what the crew will do?

Light aircraft rarely have more than 1 Altimeter

@Jamiec The PA28s I fly have two altimeters. The secondary one is tucked down next to the ammeter and mags, and takes its input pressure from inside the cockpit rather than the external static pressure tube.

Since you describe a captain and FO, I'm going to assume that you are asking about a larger aircraft, like say an airliner or business jet.
Part of the certification of these types of aircraft require them to have a third (standby) altimeter in addition to the two primary altimeters.
When there is a discrepancy, the typical procedure (keep in mind that a pilot should always follow the manufacturers checklist in these situations) is to cross-check the three altimeters and find the two that agree so that the bad one is then excluded.

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How to control fear of flying?
Four months ago I travelled to US (my 3rd time visit) from India. I always have that aerophobia just before and during flying but it has grown exponentially when the American Airlines flight took-off from UK (connecting flight) that shook a lot on runway and making lot of noise from inside (probably the cabinet doors). This made me very scare because I never had such experience and after it tookoff into the air it was shaking a lot and moving slightly up and down, may be because of heavy wind. I was so scared and thinking there could be something wrong. My BP raised when I saw lady sitting next to me. While I was trying to stay calm and act like normal, looking at her facial expressions it gave me strong feeling that something is wrong and I was watching outside from the window and then flight took a sudden turn to the left which I felt like the plan going to crash and again that body language of women sitting next to me multiplied my anxiety.
It took me nearly one hour for me to settle down. I drunk 4 pegs of whisky and went to semi-sleep for around 6 hours. Every time plane shook I said to my self "If flight crashes, I am not going to get injuries or handicapped, I will only die". I could not forget this experience even after I reached home in US, for nearly 1 month I use to get nervous about thinking of flying back. I even avoided Dallas-Vegas-Dallas trip by flight by convincing my friends that it would be fun.
I managed to settle my self only after I half-travelled back to India and said "Nothings gonna happen, thousands of flights fly everyday".
Are there any techniques people adopt for controlling this fear? While travelling back I noticed a crew member closed her eyes at the time of takeoff and landing. Does it help anyway? Was that bad experience for sitting on the last second row?

You could begin to trying to rationalise it.  It is true that the safest part of your journey began when you got into the plane, by many orders of magnitude.  You are more likely to die driving to the airport, or in a taxi or even walking there.  The other thing to remember is that 99.9999% of aircrew, who fly almost every day - often several times a day, retire or change professions.  They do not die.  Flying is amazingly safe.

Sound advice, @Simon, unfortunately, phobias are irrational fears that go against every bit of sound logical thinking. I'd venture that our OP would be better off visiting Cognitive Sciences.SE with this question, because it's more of a psychological than a flying issue. We all know how safe flying is, (s)he's just afraid anyway. That said, the best way to conquer a fear is, generally, to face it, not avoid it.

I'm voting to close this question as off-topic because it's really about treating a phobia and that isn't something that aviation experts can address. By analogy, someone who has an irrational fear of spiders should talk to a medical professional rather than to a biologist.

@Pondlife. While I agree on the general idea, including for the spiders, I remember that airlines organize simulator sessions for public to help them manage their flying fear. Air France (unfortunately in French): 7h30 training, and 30 mn with a psychologist. There are other, including non airlines organizations.

@mins Yes, that's a good point. British Airways used to do the same thing, I think. Maybe they still do.

I am fine if this can be moved to  Cognitive Sciences.SE like @FreeMan said

This is a quite good question but I think it should be asked either on travel.SE or on cognitive science.SE as suggested in other comments.

That is insane to close a question related to aviation. Not everybody can be an expert, especially the people who are not used by plane transportation but still they board one. So their "experience" is legitimate, thus could ask a question about things related to aviation including fear of it. So wrong.

The best advice I can give is:  First, do your best to learn about how an aircraft works, what happens in the cockpit, and what causes certain phenomena (eg. turbulence) which you might notice in the cabin.  Second, give yourself experience in flying as a passenger, preferably from a more comfortable seat (further forward) than one at the very rear of the plane.  At some point you might find yourself reassuring other travellers!

@Chromatix What makes you say that the front seats are more comfortable? Wouldn't the middle be best where you cannot sense pitch shifts as much?

"Flying is amazingly safe" is what echos in my mind now. @Simon 's comment helped me to get relief from fear. I took more than a dozen flights since my last post this including 3 trips to US and I am much more comfortable now. I owe Simon tons of thanks.

@Cloud I said "further forward" rather than "right at the front"; economy class usually extends as far forward as the leading edge of wings, maybe a little further depending on aircraft and airline.  But the foremost seats are still much more comfortable than the rearmost ones, because the movement of the nose in turns and turbulence is more natural than that of the tail.  Why else would they put the most expensive seats there?

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What's the technical name of the side window on Boeing and on Embraer?
On Airbus, the window beside the Captain and the First Officer has the technical name of SIDE WINDOW.
What's the name on other aircraft, or they have the same specification?

I fail to see which window you are refering to. You may add a picture highlighting it.

"Side window" sounds like a purely decriptive term, to me, and not a technical term.

Adding to @DavidRicherby comment, other "names" exist on Airbus documentation: What is this pattern on the A350 front window?

The windows to the side of the pilots on the Embraer airplanes are called "Direct Vision" (DV) windows.   They are named as such because they can be opened and removed both during normal ops on the ground and emergency ops while airborne to provide forward vision when the main windscreens are obscured.

For Boeing, the captain's side window is the L2 window, with L1 in front of him, R1 in front of the coplilot, and R2 being the copilot's sliding window. The L3 and R3 window are aft of the sliding windows.


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What does ATC mean when they refer to "company"?
Listening to LiveATC there are references by various ATC entities (towers, approach, etc) to "company". To what does this refer?

Generally when used in ATC movement instructions the word "company" means "The other aircraft operated by your company".
In this context it's a convenient shorthand for controllers and pilots: If two aircraft from the same operator are going to the same place on the airport (one may be told "Follow company"), or to assist with visual separation and sequencing on approaches.
In other cases it can mean "the airline (company) you work for" (such as being instructed to taxi to a ramp and "call company for your gate assignment" - the distinction is made based on the context of the request or instruction).

Note that this is not standard phraseology: despite being widely used the term company does not appear in the pilot/controller glossary or the Air Traffic Controller's Handbook .

@Articuno I haven't seen it documented, but it is common knowledge in the industry. I have even heard it used when referring to other aircraft in a flight school fleet, although that felt kind of odd.

I fly out of a flying club.  Upon returning to home base, I have had Ground tell me to "hold for company traffic right to left on Alpha", referring to another plane in our club.  Clearly the definition of "Company" is informal.

Is this a North-Americanism? Is it used in EU and rest-of-world? (Q isn't tagged FAA)

There are also discrete "company frequencies" which are assigned to (some?) airlines for letting aircraft communicate with some airline ground facility. I know of at least one where the call sign of the ground facility is simply "COMPANY".

@RedGrittyBrick Heard it from ATC in the UK referring to a flying school trainer, so not just North America, and not just airlines.

@abelenky Yeah they do that at my home field - sometimes it's obvious (All the Farmingdale State College planes look the same, all the Ventura Aviation planes have the same scheme with different tail colors), other times it's not (the Nassau Flyers fleet has a bunch of planes on leaseback with varying paint schemes). Usually you can figure it out, but if you don't know the fleets it can be confusing for a new student.

Yes, JAL trains at KAPC in twin pipers, and ATC routinely calls them "company traffic" to their brethren.

re: "the distinction is made based on the context of the request or instruction)." – shouldn't ATC language be unequivocal ? I thought that was one of two of its hallmarks: short and unequivocal.

@summerrain One example refers to company planes, the other to company dispatchers.  Both obviously reference the same idea ("the company you work for") applied in a way that makes sense in the given context, so there's no ambiguity.

It means, "The same airline as you".
The typical communication is:

"Cactus 123: You are cleared to land, 31L. Following Company Traffic on a 2 mile final"

Which means:

"You are following another US Airways flight, that is currently 2 miles from the runway"

It helps the pilot identify the airplane out in front of him. For example, if he spots a Red and Blue Southwest plane, he knows he's not looking at the proper plane!  He knows he needs to be looking for another US Air paint scheme.

So, when an emergency aircraft is attempting to 'figure out where to go" when touched down (with a low tire, but emergency services determined the plane could be moved) and the PIC is asked by Ground "do you need to call company", the pilot whips out his iPhone?

That is an entirely different context, (and outside my expertise).  The PIC has a different communications channel to his company's dispatch center.  (I think it is a separate radio frequency, but I'm not sure)

@abelenky: It can be another fm radio, shortwave radio or satellite phone and possibly other things, depending on what's going to work in given conditions. On the ground, cell phone is an option too, in the air they may not work as GSM has problems with high speeds.


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How far can A&P mechanics operate aircraft under US regulations?
Anytime an A&P mechanic wants to test the updates/fixes made to an aircraft's systems or any part for that matter, what are the laws (at least under the FAA) that legally allow them to operate an aircraft?
I've personally seen GA airplanes being taxied (in non-movement areas so far) by A&P mechanics. Is there a law that states that they can/cannot operate that very aircraft on a taxiway (movement area)?
What about larger aircraft (business jets, airliners etc.)? Do authorized mechanics always need a licensed (and appropriately certified with the required ratings) pilot all the time when testing the aircraft after repairs?
Finally, what about military jets? I once acquainted with someone who worked on military jets' electrical systems, but was never licensed to fly. Would he solely rely on the PIC's conclusion about the condition of the jet?

I'm not sure there is a regulation for it, but as far as I know, pretty much anybody can taxi an aircraft around, movement area or not. Military is a little different, they aren't going to let some mechanic fresh out of A-school move a $100m jet around. Most companies won't allow this though, there have been some accidents involving mechanics moving aircraft, like this one.

The laws for an A&P are the same as they are for a pilot. To fly the aircraft you must cary the appropriate licenses and ratings required for the airframe. No special cases are made for mechanics even for return to service flights.
However mechanics often fire up the engines and may run them up outside of the hangar to test things. As far as I know no certified pilot is required to start up and run aircraft engines.
The part about taxiing is partially addressed here. I know that mechanics can get some kind of approval to taxi aircraft if they do not hold any licenses, when I flew out of KPNE I knew some of the mechanics on the field and they needed to get some training on radio operations (as it was a towered field) before being allowed to taxi aircraft.
The last limiting factor, that may be a bit more ambiguous is the insurance regulations. For example say you were to own a little C172, your insurance may stipulate that only you (and perhaps an instructor) was allowed to fly the aircraft. In this case it would be unwise for your mechanic even if properly rated to fly the airplane as your insurance policy does not cover that.


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Where can I find an image of the Thales FMS220 RMS page?
I'm looking for photo/color image showing Thales FMS220 RMS (Radio Management System) page. Also COMMUNICATION and XPDR pages are highly welcome.

Have you tried contacting Thales directly?

@FreeMan - Yeah, mail was send, still waiting for any answer. But probably, if i'm not an air company, it is hopeless.

@mins - There is one user manual on different servers, but not including these pages. Probably ask reali life pilot to take shots is only option.

@Robert: That seems a good question, at least to me and another user ;-)

The answer is on the official website, here is a direct link:
https://fr.scribd.com/doc/204747124/Fms-User-Guide
You need to go to the website yourself because the document content is not to be disclosed without official approval.

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text:
What is the need for TCAS?
Isn't it the job of the air traffic controller to assign altitudes and make sure airplanes do not collide in mid-air? Is having TCAS mandated or optional? If it is a mandate, then is there a mechanism to notify other airplanes that TCAS is off?

There are regions in the world without ATC...

Because otherwise this could happen: http://www.bfu-web.de/EN/Publications/Investigation Report/2002/Report_02_AX001-1-2_Ueberlingen_Report.pdf

TCAS is used to improve the safety, not to control aircraft.

Relying on ATC and not having TCAS requires all three of these: 1.  That you are under positive ATC control.  As @sweber mentioned, this is not always true.  2.  That ATC will notice that one or both aircraft with a potential conflict are on the wrong heading or at the wrong altitude. 3.  That if the controller doesn't notice, the ATC system generates an automatic conflict alert.

A few mid-air collisions where a functioning tcas would have saved the day: Gol 1907 (154 dead), Chakhi Dadri crash (349 dead), and the Uberlingen disaster that Federico mentioned (71 dead, 72 if you count the ATC that was murdered)

Actually in the Uberlingen crash tcas was working and would have saved the day had they followed it's instructions.

TCAS and similar technology which allow interaction between aircraft maybe an important component of future ATC, like car networks in the automotive field. A more hype name would be internet of things.

TCAS is used as a last line of defense to avoid a collision. An air traffic may be distracted or occupied elsewhere and so no instruction is given to the two aircraft. The TCAS overrules any instruction given by air traffic control and the pilots must specific that they are following TCAS over the radio. For example if an aircraft is instructed to descend by air traffic control and TCAS is requesting the aircraft to climb the aircraft will begin a climb. TCAS will order one aircraft to climb and the other aircraft to descend to safely end the conflict between the two aircraft.

TCAS is part of a defense in depth strategy to reduce air accidents. ATC does not cover all areas, so TCAS adds protection where ATC coverage is missing. ATC is not infallible: humans can make errors and computers can go down. With TCAS pilots can get life-saving warnings even in zero visibility, without reliance on any ground based system.

It is the job of the ATC to assign the aircraft altitudes and ensure their safety. But consider the following situations:

There is no ATC coverage.
(one of the) The aicraft fails to respond to the ATC directions.
The aircraft follows the ATC directions, but there is an error either in the aircraft systems/pilot actions or in the ATC directions.

In these situations, the TCAs acts as the next layer of defence against mid-air collisions, acting independently of the ATC.
Most of the regulatory authorities (FAA, EASA etc) mandate TCAS (usually TCAS II) in the civil aircraft beyond a certain capacity, though the requirements vary from country to country.

And just to add a specific scenario (although it's covered in your second point): an aircraft's radio may fail, and therefore they can't receive ATC instructions. ATC can try to direct everyone else around them, but only if ATC notice (unlikely until they need to contact the aircraft). And if the aircraft can't tell ATC their intentions, ATC may find the aircraft without comms changes altitude into the aircraft ATC just directed around them!


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Why were square windows a problem for the Comet, but not for the Super Constellation?
Famously, one of the problems dooming the Comet was the square windows. But the contemporary Super Constellation also had square windows but I can't find any articles detailing similar problems to the Comet. What was the difference?

Image source
The difference was in the cruise altitude of the two aeroplanes. The Super Connie was powered by piston engines and had a service ceiling of 25,700 ft, usually cruising at about 20,000ft as mentioned in this article about the Connie. The Comet cruised at an altitude of 42,000 - 45,000 ft, with its fuselage pressurised at equivalent altitude of 8,000 ft.
The much larger pressure differential at cruise altitude added up to fatigue cracks in the fuselage. Not necessarily at the window corners, as per the wiki:

…. the fuselage had ripped open at a bolt hole, forward of the forward left escape hatch cutout. The failure then occurred longitudinally along a fuselage stringer at the widest point of the fuselage (accident report Fig 7).

Reconstruction of one of the wrecks revealed a fatigue crack starting at a rivet hole, leading to a change in riveting practice on the Comets - no more punch riveting, the holes were drilled.
The Comet was the first airliner with jet engines, enabling it to cruise above the weather. But unfortunately it was also the first airliner to experience metal fatigue from repeated cycles of pressurisation.

They'd originally planned to glue the skin to the frame, but this was overruled, hence the original rivets.

While the cracks on G-ALYP did start at a punched rivet hole, this hole was near the corner of a square window on top of the fuselage. Stress concentration there helped those cracks to grow most quickly. The Wiki report concerns G-ALYU which was tested later in a water tank.

Wait, the Comet was only pressurized to an altitude equivalent of 20,000 feet? How did people breathe and maintain consciousness? Don't modern regs require oxygen for unpressurized flights above 10k feet?

Regarding "not necessarily at the window corners", the german wiki says the myth comes from the final report using the term "window" for the cover of the radio compass and had absolutely nothing to do with the passenger windows. It was just an unrelated part of the investigation that square windows are probably also not ideal.

@FreeMan it appears that the Comet was pressurized to 8000 ft https://zenodo.org/record/2551089#.YgaCb_nMKUk

Also, OT, but I love how that shot lined up so the arch in the Connie is almost perfectly mimicked by the arch in the BUFF's wings behind it.

@Freeman Indeed, corrected.

@FreeMan One can breathe and maintain consciousness at 20000ft. There is a big rick of the acute maintain sickness if you are there for too long, but you are not going to faint. Take a bus ride to Leh and you will cross several cols around 5500 m above the sea level. It is routinely done by ordinary people without any previous acclimatisation. If you fly to Leh by a plane, you will get out at above 11000 ft and stay there.

There is a huge misconception here.  The problem was NOT window shape in itself.  It was errors in the calculations that determined the material and fastener requirements for the window frames and local structure as used, combined with the lack of a long term pressurization fatiugue test before entry into service that would have uncovered the error.
They were simply under-built for their particular frame profile and the pressure differential being used.  If you made the corners beefy enough, with sufficient material thickness and enough rivets, you can meet fatigue strength requirements with any shape.
At the extremes, you could make window frames from heavy, thick high strength steel, and use a window with razor sharp corners like your house, if you could live with the weight.  At the other extreme, if you want the lightest possible structure, you make the windows perfect circles so that almost all the stress around frame is pure tension.
So the bottom line is the Connie's windows had adequate skin thickness, frame cross-section, and fastener type and count for the stress levels imposed by its pressurization requirements, and the Comet didn't, due to undetected calculation mistakes.  Had the proper calculations been done, DeHavilland would have either increased the corner radius, or added more rivets and material if it could live with the weight increase. In the end it chose to go with oval windows to avoid the weight increase (and probably an abundance of caution along with avoiding the whole "square window" meme that had taken hold by the media coverage), but in theory it could have simply reinforced the corners by whatever it took.
Modern airliner windows are also mostly square, but with a larger radius at the corners and a larger vertical dimension than horizontal dimension, which minimizes the "missing meat" in critical areas of the fuselage between the windows, and this allows the best compromise between pleasing window shape and adequate structural margin while keeping the structure reasonably light.  Make the frame too thin or undercount the rivets, and you'll have 737s or A320s blowing up from window corner fatique as well.

They went with oval windows and also beefed up the structure and the skin thickness considerably for the Comet 4. If you ever can visit the De Havilland Museum at Hatfield you can compare them practically side-by-side.

Sounds like a destination for my next UK trip!   That just shows how far off their stress calcs were.  But to my point, they could have beefed things up even more and stayed with squarish windows if they really, really wanted to (say, due to a passenger appeal imperative that trumped everything else).  The Avro Jetliner in Canada was developed concurrently with the Comet (first flight was about two weeks later) and Avro Canada elected to go with round windows from the get go.  Too bad they only ever built one.

The hull openings on the Comet were cut with hydraulic shears, which introduced stress raisers all around the periphery with each jaw closure. Square windows then focused the overall stress distribution at the window corners and where the corners coincided with a "sknitch" produced by the shears, you'd get fatigue cracking.
After the Comet crashes, Boeing and Douglas switched to chem milling of hull panels (in which a hull panel was painted with resist except for the hole areas and then dipped in a chemical bath until all the unprotected areas had been etched through and eaten away) This produced atomistically smooth edges without shear damage. I'm willing to bet that Lockheed chem milled the Constellation's hull panels too.

authors: Vladimir F Героям слава


text:
What causes tail-down force?
What phenomenon causes aerodynamic tail-down force? Why will there be more downforce on the tail when an aircraft picks up speed? What happens if the aircraft goes too slow - does this means less downforce on the tail? Do the center of lift and center of gravity play any part in determining the downforce on the tail?

The exact same phenomena by which the wing produces lift that varies with speed and angle of attack.  The tailplane is a wing.

Do the center of lift and center of gravity play any part in determining the downforce on the tail: not the force, but what counts here is the moment of this force related to the CG (think of a lever). The moment depends on the distance between the tail and the CG.

On a conventional wing and tail configuration the center of lift is normally behind the center of gravity. This causes a pitch-down force, which is counteracted with the stabilizer's downward lift.
As speed increases, the wing produces more lift, thus more tail-down -force is required to maintain attitude.
On a rear-wing configuration the same is accomplished with a canard. The only difference is that the canard provides upward lift and so it adds to the total lift, contrary to a conventional tail just increasing drag.

The phenomenon is called trimming.
This is the process by which the pilot adjusts the lift at the tail surface to shift the center of pressure right above or below the center of gravity and removes any stick force so the airplane will keep this attitude unaided.
If the center of gravity (CG) is right above or below the aerodynamic center (AC), the aircraft's longitudinal static stability is indifferent. Move it forward and stability increases, move it further back and the plane becomes unstable. The aerodynamic center is where the angle-of-atttack-dependent lift forces act, and in an indifferent configuration the lift coefficient on wing and tail will be equal. Increasing stability thus means to reduce lift at the rear surface, and if a lot of stability is desired, lift at the rear surface will become negative.
When the airplane is hit by a gust (change in vertical wind speed), the angle of attack at both wing and tail will change by the same amount. Now it is important how much lift changes relative to the lift before the gust. If the lift coefficient on the wing is larger than on the tail, the same amount of angle of attack change will cause a relatively smaller change of lift on the wing than on the tail, and the center of pressure will move such that it causes a correcting pitch moment.
By selecting the proper center of gravity location, the pilot can adjust the level of stability he/she desires. To stay at the desired attitude in flight, he/she now has to move the incidence of the empennage (or the elevator deflection) to a point where any pitch moment disappears. The consequence is a downforce on the tail if that is what it needs to trim the aircraft with the given center of gravity location.

The tail is a lifting surface just like the wings. Downforce is just the phenomenon of lift directed in the opposite direction. You can also see examples of downforce in things like the spoilers on a Formula 1 car. Going faster produces more lift and so more downforce.
What's unique about the tail is that it can be affected by the downwash of the wings themselves which comes into play when looking at design choices like a T-tail.
Zooming out more broadly, the reason you want downforce is longitudinal stability. On some planes and in certain configurations the tail produces positive lift instead of downforce, all in the name of balancing the plane, though those designs tend to be harder to fly.
You may notice that the tail is usually much smaller than the wings. This allows for better longitudinal stability as the tail experiences larger relative changes in lift (downforce) compared to the wings which provides a negative feedback loop that brings the plane back to equilibrium.
The location of the center of gravity relative to the center of lift matters because it produces a certain pitching moment that then needs to be counteracted by the force produced by the tail.

this answer misses the central point that the downforce is caused by the tailplane airfoil being "upside down"

Thanks for the feedback, you're right. I tried to clean up the answer a bit and make sure the specific question was answered first.

@rbp: No, the airfoil is only a consequence. The real cause is stability, as Miron says. Putting a negative camber on the tail will only make it more efficient - the size of the downforce is not affected.

Maybe this is a language issue. "Consequence" means it happens because of some cause.


text:
What is the role of a flight control computer during a steep approach?
I am working on a flight control computer (Bombardier) where I got a bunch of requirements for steep approaches. All they do is 'engage steep approach' or disengage it after determining if it is safe to engage the steep approach.
Even after engaging the steep approach, if the pilot does not configure the flight properly (throttle, flap, slat settings) then it disengages the steep mode and informs the pilot.
After carefully going through the requirements, I did not find anything that the flight control computer does during the steep approach.

So is all that the flight control computer does during steep approaches to monitor it and display steep engage or disengage messages?
What would a pilot do if steep approach was to disengage when he reached below 200 ft? (Breaking off from steep approaches, I understand, is the result of losing some of the sensors, jamming of actuators etc..). Can a pilot 'go around' even if the altitude drops below 200 ft (decision height of CAT I landing)?
Is there a possibility for the aircraft to stall while trying to descend steeply?

Which Bombardier product?  Learjet 45?

Bombardier C-Series 100

I can't answer the question, but it sounds very similar to requirements for Cat II approaches.  Just a bunch of monitoring and handling the annunciators.

I hope you get some useful answers here, but I also hope you ignore them all and ask for your requirements to be updated with the answers :-) More seriously, anything you learn here may be very useful background information for you, but your requirements should be the final reference.

Sure,my question is - does pilot get any   assistance from flight control computer or autopilot or he just hand flies the airplane. If he need to do everything by himself it seems like lot of work to me

Well my question is, does the flight control system in general provides any protection mechanism for the aircraft performing steep approach. Or it just tells the pilot that the aircraft configuration right now is 'okay' for performing steep approach and pilot has to do everything normally. I know that autolanding can not be performed using steep approach. So is fully the responsibility of the pilot while descending from around 2000 ft at glideslope angle of around 6 degrees to make sure he does not stall the aircraft/ overspeeds the aircraft etc.., ?

Steep approaches may be referring to how the Flight Control Computer is controlling aircraft speed. Some aircraft provide two types of speed control depending on the flight control mode.  One where speed is controlled by pitch and one where speed is controlled by thrust.  The two modes are front side and back side (of the power curve).  http://joeclarksblog.com/?p=771 explains it pretty well.

On the Airbus 318, it appears that the flight control computer does more than just monitor and handle annunciators.

In March 2006, the European Aviation Safety Agency (EASA) certified a modified control software enhancement to the Airbus A318 designed to allow the aircraft to perform steep approaches. The aircraft is the largest commercial aircraft certified by EASA for steep approach operations. The software modifies the control laws of the aircraft when the steep approach function is selected by the crew, by automatically deploying some of the spoiler panels to provide additional drag when the aircraft is in the landing configuration. It also provides alternative aural alerts to the crew and modifies spoiler deployment automatically below 120 feet (37 m) on landing. The A318 steep approach procedure allows the aircraft to perform approaches at descent angles of up to 5.5°, as opposed to the standard 3° for a normal approach.

A318 Wikipedia
The answer to your question #2 can be found in the aircraft operations manual.
The answer to your question #3 is that an aircraft can stall at any attitude and configuration. A stall is when the critical angle of attack for the wing is exceeded.


text:
What is the way to calculate the required twist of a flying wing?
I am interested in flying wings and a couple of weeks ago I started researching them. I found mh-aerotools.de website and think it's a great source.
The website stated that flying wings need twist to stabilize, but I don't know how to do this or even is it required at all.
Is a twist needed and if so how do I to calculate it?
Note: I chose airfoil EH 1.0-9.0 for this purpose.

Related: Twist Distributions for Swept Wings and The Culver Twist Formula. It's RC-applied.

Twist is closely coupled to the wing's sweep angle, airfoil camber, taper ratio and the desired level of static longitudinal stability. Other factors are the desired spanwise lift distribution and maneuverability. There is no simple, general formula: In the end twist, or, more precisely, local reflex camber, is the consequence of your selection of the parameters mentioned above.
Background
Static longitudinal stability means that the aircraft will return to the trimmed angle of attack after a disturbance. This is made possible by producing proportionally more lift in the forward part of the wing than in the rear part. Reflex airfoils do it all by themselves and unswept flying wings do not need washout, but a proper reflex airfoil.
The Horten flying wings had highly tapered wings and used a bell-shaped lift distribution over span which produced a small downforce at the tips. This helped a lot to reduce adverse yaw and allowed them to do away with a vertical tail. Also this made stall chararcteristics benign. This lift distribution was reached by extensive washout (as much as 8°).
If you add stability by computer control, washout will not be needed and you can use an elliptical planform for best performance. This will, however, have unfavorable stall characteristics and needs wing fences when the wing is swept. Moderate sweep adds stability and damping, but above a critical combination of aspect ratio and sweep angle it is hard to achieve satisfactory stall characteristics.

Critical angle of sweep for swept wings, from Chapter 16 of S. Hoerner's Fluid Dynamic Lift. Too much aspect ratio and sweep will result in a strong pitch-up when the wing stalls.
By adding some chord at the outer wing you can twist the local incidence by the amount of chord added. This will lower the local lift coefficient and produce reserves at stall which will improve stall characteristics a lot. Karl Nickel has published a method for calculating the optimum planform for a given static stability with nearly elliptic lift distribution over a wide range of speeds. The key is his insight that by trimming the wing for a particular angle of attack the pilot will adjust local washout by moving the elevons trailing edge up. Ideally, the wing uses several elevons with increasing trim deflection towards the wingtip. (By "trimming" I do not only mean the reduction of stick forces, but also the adjustment of local lift to cancel any pitching moments.)
Note that now the planform is tightly coupled to the static margin and, consequently, to the location of the center of gravity (cg). Flying with a more forward location of cg than ideal means more negative elevon deflection angles and lower local lift at the tips than ideal, and vice versa.
The SB-13 flying wing glider uses this technique and has a taper ratio (ratio of tip to root chord) of 0.8. It uses two elevons over the outer 50% of span, the inner of which travels 1/3 of the trim deflection of the outer elevon. Twist is zero for the inner half of the wing, changes from 0° to -1.5° over the 0.15 m section where the root airfoil (HQ 34 N) changes over to the tip airfoil (HQ 36 K) to adjust for their difference in zero-lift angle and increases again linearly towards the tip to +0.5° (I hope I got that detail right, typing all this from memory). The negative washout was chosen to allow the elevons to have 4° twist themselves, so in sum (fixed wing and elevons) the wing has increasing twist towards the tips. In flight, the elevons will always have a slightly negative deflection, so the effective washout is even stronger.

SB-13 in flight (picture source)
To answer your questions directly:

Is (a) twist needed?

Not necessarily. An unswept flying wing does not benefit much from twist. It will only help to add some stall margin at the tips. A positively swept flying wing can also get away without twist if the airfoil is changed over span from positive camber at the root to negative camber at the tip. If the same airfoil is used over the whole span, twist/reflex camber is necessary and cannot be avoided as soon as the cg is ahead of the neutral point: It will result from trimming the wing for the desired angle of attack.

How do I to calculate it?

Select your desired static margin.
Select your desired lift distribution over span. An elliptic distribution will result in the lowest induced drag, and a bell-shaped distribution will give better maneuverability.
Select the planform that results in the desired lift distribution when trim changes over speed are considered. Use several elevons over span to be able to adjust the local incidence independently.
Twist is then a necessary consequence of trimming the aircraft.

Thanks. very useful. Q1: You say "twist is necessary as soon as the cg is ahead of the neutral point" so can I ignore twist because cg is ahead of np?  Q2: How do I select lift distribution over span? Q3: By trim you mean the trim tab in aircraft?

@SAMPro: If you ignore twist, the resulting elevon deflection might be excessive and the laminar bucket and maximum lift coefficient of the airfoil might be shifted to an undesirable location. It is better to consider it from the start. If you don't, it might hurt you later. Lift distribution: That is up to you! You need to get your priorities right, then the right lift distribution becomes obvious.

@SAMPro: Even if I put it in brackets, this sentence is important: By "trimming" I do not only mean the reduction of stick forces, but also the adjustment of local lift to cancel any pitching moments. And you adjust local lift by selecting the proper elevon deflection.

I'am an amateur just want to fly my craft, so I don't have any priority. My only priority is flying my tailless soaring plane. Is there any suggested values? I can't find a well defined one.

@SAMPro: Build a model first. Use the bell-shaped distribution as used by the Horten brothers. Make sure that your plane has a stiff wing, so coupling between the fast period mode and wing bending occurs above your speed range. The SB-13 used 12° spar sweep in a 15° swept wing for aeroelastic tailoring and high modulus carbon fibers. If you want to build a tailless plane, you need to understand what you are doing. Just ground-handling the SB-13 is a science in itself. It's much easier to add a tail - that solves a lot of problems.

There is not any need to use twist in a flying wing aircraft IF the chosen profile is already reflexed. The use of twist will generate a very reduced optimal speed envelope and it will create as well a repetitive longitudinal phugoid motion (typically lasting at intervals of 7 seconds, in full size craft), similar to porpoising in watercraft.
A good reflexed profile or a pertinent series of profiles progressively changing form root to tip will perform better. The basic idea is to have a profile/s in which the position of the Cm will not be excessively altered with changes of AoI (or AoA when in flight).

You could maybe add that twist only helps with swept flying wings. And please elaborate why twist would cause a phygoid motion! Does it involve aeroelasticity? And the 7 seconds are only true at one particular speed.

Twist does not only help with swept wings. The phugoid motion will happen at any speed which is not the one at which lift and twist action will balance each other. That is the reason for which twist reduces so much the usable stable speed envelope.

The wing twist (washout for swept back wings) helps in speed and stall stability.
Consider a normal aircraft- i.e. the ones that have a tail (horizontal stabilizer)- in this case, the wing produces an upward force, while the tail produces a downward force. Here, as the nose goes down, the speed increases, and the lift produced by the main wing means that the nose pitches up again. The opposite happens when it the nose goes up- the speed drops and as the airspeed decreases, the nose drops (as lift is now lesser). As the wing is at a higher incidence compared to the tail, it stalls first, and the stabilizer (which is still producing lift) pitches the nose down, bringing it out of the stall.
As there is no horizontal stabilizer in case of tailless aircraft, these effects have to be built in into the wing itself. One way to do so is to have a washout- having a swept wing with the outboard area at an incidence lesser than the inboard (root) section.
In this case, as the speed increases, the front portion of the wing (near the root in swept wing) produces more lift as the speed increases, pitching up the nose. The opposite happens as the speed decreases- the nose pitches down as the lift is decreases with airspeed loss.
As the aircraft approaches stall, the inboard section (at a higher angle of attack) stalls first, while the outboard section, which is at the rear (due to the wing sweep) causes the nose to pitch down, causing the aircraft to come out of stall. The added advantage is that the control surfaces are still functional.
The same effect can be obtained by using a reflex airfoil and an unswept wing, with the reflexed portion acting as the tail.


text:
What is the best design for a small flying wing with special flight envelope?
I'm designing a small, light and portable flying wing UAV that will launch and land vertically via a Tilt-rotor system. I'm attempting to use the program XflrV5 but am having difficulty understanding the numbers. I'm a design and electronics guy, but a poor engineer.
The craft has a span of 6', root chord of ~8". Since it is a VTOL it does not need to fly slowly - below 15-20 MPH. I would like a top speed of ~100 MPH. the end product will be likely built with carbon fiber so compressibility shouldn't be a problem. After some reading I am tentatively using the s5020 airfoil but I'm not married to it.
My primary priority is forward flight efficiency, second priority is reasonable stability as it for recon use (forest fires, search/rescue military) but this can be mitigated by use of a stabilized camera gimbal. Maneuverability is a low priority if it has to be to accommodate the above items.
My question is with these parameters what type of wing design would work best and would sweep and twist be needed considering the speed envelope?
A UAV with a similar envelope would be the Boeing Scan Eagle as it is catapult launched and flown into a net for retrieval.
-------------------EDIT 1 -----------------------
I have included some pics of possible designs  to help show some ideal designs to meet the portability requirement. Note: the outer 2' (660mm) of the wing will be removable for portability.
It would be a great help to hear your opinions which would be best.

-------------------EDIT 2 -----------------------
A similar model to mine is the Firefly6... mine will have some improvements as well as a different market. I forgot to add that the AUW will be 6 - 7 lbs. (~3Kg)

an earlier post by me was flagged as having too many questions so i reposted this one with a narrower focus. thanks!

Looks good to me now Jeff.  Let's hope that someone on here has the expertise to help you.  Peter Kampf will probably pass on by soon...

I know, flying wings are cool. But once you have built and flown one, you get a more nuanced view. To find the right configuration, start with a look at your objectives. You want:

excellent performance and
an observation platform with good stability.

Now let's see what a flying wing will do for you. Apologies for the heavy use of technical terms; please follow the links for more verbose explanations:

It will have little pitch damping, so your airframe will wobble through the air with every gust or control command. You will need wing sweep to get at least a little aerodynamic pitch damping.
Depending on wing sweep, wing spar stiffness and pitch inertia, the frequencies of the wing bending oscillations and the fast period mode might converge within the desired speed range, resulting in flutter.
With a span of 2m and a root chord of 20cm the aspect ratio is above 10. If you sweep the wing, your low speed characteristics become outright nasty. I know, you will use engine power for take-off and landing, but once you fly a tight circle, you will bring the wing close to stall even at higher speed, and then a high aspect ratio swept wing will start to roll on its own and pitch up. In other words: It will become uncontrollable.
A reflex airfoil only produces lift in its forward part. The rear part produces a little downforce at high speed and proportionally much less lift than the forward part at low speed, so the whole wing will become less efficient. It will be better to reduce wing area and to use an airfoil with an excellent ratio of lift to drag.

My advice: Put part of the saved area at the end of the fuselage as a horizontal tail surface. This will give you ample pitch damping (goes up with the square of the tail surface's lever arm) and enables you to use a high-performance airfoil on a very small wing which produces much less drag than a flying wing of the same minimum speed. If you want good high-speed performance it is important to keep the wetted area at a minimum to minimize friction drag.

Now that you have expanded the question and added portability, let me propose something that would combine all three characteristics: The Leading Systems Amber. It was designed to fold to the size of a torpedo to be carried onboard Navy ships by pivoting the wing by 90° and folding the two tail surfaces forward. Combine this with a folding propeller and hand-launch it to avoid conventional take-offs.

Leading Systems Amber in flight (picture source)
Landing was done by deep-stalling the aircraft. To quote from an article about Abe Karem in Air&Space magazine:

Most ingenious of all, Amber could be recovered by putting it into a
  “deep stall”—a feature used in free flight glider models to escape
  thermal updrafts and stay within flight time limits. The stall would
  bring Amber to “a near-vertical landing so it could be used from small
  ships, submarines or trucks/trailers,”

At least, this is what I would do: Build a scale model of Amber.

Hi Peter and thank you very much for your time. for some background, I own an RVJet which flies great but needs an enormously long landing approach due to a large ground effect ( Ive flown it 100 meters at 1 meter in height before it touched down). this is what spawned the idea for this plane.     Im wondering if using a wing like this ( a slight delta) would qualify as your solution?   thanks

sorry for another comment... i am trying to use a tailless plane so it can be put inside of a backpack. thanks

I have edited my original post and added some pics if that is OK to do...Jeff

@jeffdelvecchio: Yes, it's OK and helps me to understand that portability is also a design goal. Now the hardest question is how to combine the flying wing with a tilt rotor. That's not easy!

Hi Peter, the Pixhawk autopilot can now handle the transitions fairly well and its getting better. The prop configuration is basically a tricopter layout where the 2 front props tilt forward/up and the 1 aft prop tilts port/starboard for hover yaw control and turns off in forward flight. there are other control schemes available and there are 2 similar models... one that is very similar is called the firefly6. thanks

added a pic of the firefly and my AUW of 6-7 lbs (~3Kg) to the main topic

Thats a very interesting design Peter! I like the pivoting wing... it solves some design issues and allows for a lot of efficiency..... I assume a plank wing does not need twist or reflex?   thank you for this....Jeff

@jeffdelvecchio: Twist is needed depending on taper ratio and the design lift coefficient. In the end it is a compromise between stall characteristics and performance. Amber had no twist and a constant wing chord so it was stable when in deep stall. No reflex airfoil needed - just pick what gives best L/D.

Don't forget that flying wings are hideously unstable in yaw; that low vertical keel I think I see in your sketch is mostly near the midpoint of the craft's length and will add little if any directional stability.


text:
Why don't black boxes send a signal when above water?
Cockpit Voice Recorder and Flight Data Recorder's "Underwater locator beacons" only send their position when underwater.
Why don't they do so when not underwater?

Yet another change-to-come.

If it is a change to come, I wonder why they made it is this way in the first place. It must have some reason. I mean, detecting whether it is underwater, perhaps by measuring the pressure, isn't for free, and it seems to me they could easily have omitted it and got an even better findable blackbox. I'm confused.

The black boxes themselves don't send their position, the ELT does and it is a separate box which is used to find the aircraft, not the black box.

The underwater pinger is acoustic on ultra sonic frequencies. That doesn't travel very far in air. They are activated by a water contact switch, not on pressure.

Related, but not quite a duplicate: Why don't black boxes have their own GPS?

Are you wondering why they don't have it on the black boxes to locate the black boxes specifically, or why they don't have it on the plane to locate the plane crash in general?

See Which types of aircraft are required to carry ELTs? The sort of planes that have CVR and FDR "black boxes"  (i.e. "air carrier" certificate holders and multiengine passenger aircraft) are not required to have emergency location transmitters.

Am waiting to see a cost and or weight related answer...

The short answer is that when a plane crashes on land we (generally) have a pretty good idea where to look for the black boxes - there will usually be a relatively clearly defined debris field to search.
When a plane goes down in the water there's a lot more ocean to search, and currents can deposit parts of the airframe over a much wider area, so the underwater locator beacon ("pinger") helps guide people to the debris field.

The longer answer is that sending a signal when you are underwater is relatively easy.  The pinger has a water switch: when it gets wet the water closes an electric circuit and the pinger makes a noise that can be heard on standard passive SONAR gear.
Water is pretty good at conducting sound waves, so the pings can be heard over a large distance and can guide searchers to the black box (or at least the pinger: sometimes they get ripped off, but they're usually still near the debris field).
We get all of that functionality in a fairly small self-contained device that requires effectively no maintenance (you replace the unit when its battery expires).
Sending a signal when you're NOT under water is harder: you can activate the signaling device with a "G Switch" the same way the ELT is activated, but after that the problems get harder:

You can't use sound (effectively)
A "pinger" beeping in the air is much less effective than under water - the range would be extremely limited even at very high volumes. Add to that the fact that crash scenes are pretty noisy with firefighting and rescue crews working and the chances of anyone hearing an acoustic beacon on the surface are pretty slim.
You can't use light (effectively)
A strobe on the black box may work OK in the dark, but it's easily damaged and not terribly effective during the day, or if the recorder is upside-down, in dense foliage, under snow/mud, etc.
Radio locator beacons are problematic
Radio locator beacons require an antenna - if the antenna is damaged or torn off in the crash the effective range of a radio beacon would be drastically reduced, possibly to the point of being completely ineffective.
If the antenna remains attached the signal can still be reduced drastically if the box is buried under natural features (dirt) or aircraft debris (aluminum skin).
(Both of these issues are frequently raised in criticism of ELT technology.)

It's possible a locator system could be devised that works on the surface, but given the challenges and the fact that the location of the black box is generally known (to within the size of the debris field at least) when an aircraft crashes on the surface there hasn't been much effort invested in this.
Finally, remember that while the recorders are a very valuable part of the investigation process remember that from an investigation standpoint it doesn't matter if you find the black boxes on the first day or the last day of recovery operations:  Finding the recorders will not undo the crash or bring the dead back to life, so if it takes a little longer to find them while sorting through a debris field on the surface it's really not a major problem.

It feels like you're ignoring accidents like Varig 254 in which we had almost no idea where the plane might be, as well as the fact that most ELTs also transmit on 406 MHz in order to tell investigators roughly where the crash site is.

@raptortech97 There are several examples like that (Flight 404, searching for Fosset's plane etc.), but voretaq did specify that generally speaking, debris of land crashes are relatively on a much smaller area than crashes in water, where pieces of airplane can travel 100s of miles from the original point of contact.

@raptortech97 It probably feels that way because I am ignoring those cases where investigators had trouble finding the debris field. That's mainly because usually when a plane hits the ground we can find it pretty easily, and because there's already a technology specifically designed to help you find the debris field on land (the ELT ). You could build an ELT into each flight recorder for redundancy (or put a dozen ELTs on the plane in the hopes that at least one will remain operable after a crash)  but you'd still have all the problems listed above - just spread over more devices.

The premise is inaccurate. Modern aircraft do output a radio signal when they crash in order to guide search and rescue teams when the crash is over land.
Specifically, (some? most?) modern aircraft carry ELTs, which activate  after a crash and use a GPS unit (accurate to about 100m) to determine their location. They then transmit this information on the 406 MHz band, which is monitored by satellites.
When a signal is picked up, search and rescue teams will be notified and provided with a location accurate to about 100 meters. This allows rescue teams to find survivors quite quickly after a crash, which is vital for survival.
Note that, as voretaq said, it's not important to find the black boxes very quickly after a crash. Whether it takes 1 day or 10, the investigation will take far longer anyways. The reason we have ELTs is to find survivors. If they happen to be useful for locating the black boxes afterwards, that's a plus.


text:
Who can request a check on an engine before boarding/takeoff?
I have read on a newspage that a passenger found an engine problem on the B767 before takeoff and requested staff to check it, while the airport claims the decision was made by the pilot.
I was wondering: Who has the right to do such decision. Will the engineers check the engine just because of a passenger's complaint?

I guess you can't just call the engineers and ask them to have a look. You tell the attendants, they tell the pilot, and the pilot asks for engineers.

If the aircrew disagree that anything needs checking I doubt they would delay the take-off to ask the engineers for a check. The authority is with the aircraft captain, not with the SLF.

Seriously funny comment thread on that article! Worth the read just for that!!

I reported an issue once - thickish fluid in small volume trickling down engine outer and flicking off into space. Air hostess took it on herself to crawl over seats and look out window first to confirm my report before 'taking it upstairs'. I was careful not to let any other passengers hear my report to her. Many people were aware that she was checking out what I had reported when she did it. We proceeded. I'm still here :-).

I tried to raise an issue re passenger safety (notionally comfort but temperatures during a very long delay in sydney in summer has window seat temperatures at heart attack level for some with very hot air being pumped out of the vents. I suggested to a steward that he suggest to the captain that the middle doors be opened. He told me that this was not technically possible as under full passenger/baggage/fuel loading the aircraft would be excessively stressed. Later - maybe a very hot hour - they opened the middle doors to load more whatever (maybe food?) Lying to passengers is apparently ...

... more important than possible major passenger issues  if some inconvenience is involved. | Shame on you .... . (They usually do OK).
[[NB it was really really really hot. Really. (Maybe 50C?)(Yes, I've visited Death Valley in mid summer).  BUT mostly in the window seats - but I did not know that until I moved to make my suggestion].

LOL TransAero. If you insisted, the plane be returned to the gate and checked by an AP every time you had a flight you would probably be right half the time.

I guess this will vary from airport and scenarios. Say if you are in a small Cessna on a little grass-strip: They will most likely always listen to you. However, In a passenger jet, on a busy airport: Things fast get a little more complicated. It is very expensive for an airliner to hold a plane, and even more to ground it. And it affects more people than it would with the Cessna scenario.
If you tell, say the flight attendant while the plane is rolling down the taxiway that you hear abnormal sounds: They will likely misinterpret you as nervous and will most likely ignore it. But if you can somehow socially convince them: It is still the captain that makes the end decision about turning around.
I don't know if this has actually happened before other than in the article mentioned in the question.

If there is something wrong about the plane: The captain will likely know well before you do. The engines have so many sensors and fail-safes that the captain will likely have the fault displayed in the cockpit before you even notice something unusual, and he will probably make the decision before you even reach a flight attendant.

Some times the cases are black and white like this one where its very obvious that something is wrong (fuel is pouring out). But this brings up the question of "how do you know something is wrong with the engine?". I will admit that I know a fair amount about airplanes and avionics but I would not be able to identify an engine issue (on a jet) shy of something obviously visibly wrong. Then, as mentioned you would have to convince the crew that you knew what you were talking about. If we think about the scenario its clear that there are 2 pilots on board (who are presumably more qualified on the matter than you). 2 Pilots who should have done preflight checks and an airline that should be monitoring the use and time on its engines. So who are you to say there is anything wrong. That being said if you think there may be an issue(and have some decent grounds to think so) you should always say something as its better safe than sorry.

The pilot in command always has the final decision. Of course, an unruly passenger can always force a return to the gate. This does not mean, however, that the plane will be checked over. In general, the PIC will be glad to be alerted to any anomaly, but a passenger has no authority to demand a plane be mechanically examined.
According to a followup article, the claims that the passenger first noticed the problem were wildly exaggerated, and, in fact, according to TransAero the crew was already working on the problem when the women complained and her actions made no difference to the process. Yet another shaggy dog story?


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Is axial dynamics important for the helicopter equations of motion?
While analyzing helicopter performance, why do we always ignore the axial dynamics from the equations of motion?

Is that a homework assignment question?


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Why are liquids for cooling not allowed when drilling on CFRP/Al or CFRP/Ti stacks for aeroplane wings?
I recently did some research on the effect of cooling on CFRP/Al7075 drilling. 7075 is used for aeroplane main wing upper skin, and the plates need to be overlayed and drilled for flush riveting.
But I read somewhere that liquids are forbidden for cooling operations on these multi layer plates. Is this true? If so, why are they forbidden? Do I have to change the subject of my research?

Welcome to SE.Aviation. As an engineer, I am intrigued by why cooling fluid on 7075 would be a bad thing. However, this question as asked is a materials/machining question, and so IMHO is out of scope for SE.Aviation. Maybe ask it on SE.Engineering, https://engineering.stackexchange.com/?

@KennSebasta I think aircraft manufacturing techniques should be in scope. This is not some widely used material.

A quick search suggests in contrary wet drilling is beneficial for health when drilling CFRP, preventing small particles to be inhaled. There are also liquid coolants optimized for these operations. Can you cite or detail a bit more the recommendation to not use liquid coolants?

@mins https://engineering.stackexchange.com/questions/47466/what-is-the-reason-for-forbidding-liquids-for-cooling-in-drilling-on-cfrp-al-or

Hi Mohamad, perhaps best to add in the question that 7075 is used for aeroplane main wing upper skin, and that the plates need to be overplayed and drilled for flush riveting, so that the relevance to aircraft manufacturing becomes clear.

@KennSebesta: It would be a problem specific to aerospace, a reason not to close the question. The article is still very vague: "for most stack machining operations settled in aerospace applications, liquid cutting fluids are prohibited due to quality and automation requirements. Although alternative, non-liquid  cooling  strategies  exist,  these  CFRP/Al-stacks  are  usually  drilled  under  dry  conditions".

@Koyovis I just copy-pasted what you wrote into the question.

If this gets reopened, I'll be more than happy to copy my answer from engineering SE here: https://engineering.stackexchange.com/a/47483/35171 :)

According to this article:
A Phased Approach to Optimized Robotic Assembly for the 777X
drilling multi-material composites does require some special trickery, especially when automated assembly is part of the process:

There were lessons learned relating to process
capabilities whereby technologies were added or modified to prevent
the need to return to hole locations for further processing. Additional
cutting lubricants and methods were specified to allow layer by layer
drilling optimization specifically targeted for one-up assembly.

The cooling/chip evacuation methods were varied during single hole drilling per current material layer of stack. The reason for this is explained on the document to be:

Critical for one-up assembly as the fastening immediately follows the
drilling process, composite stacks are drilled dry and metallic
substructure layers drilled with minimal air/oil mist. This keeps the prepared hole dry and clean. To keep the evacuation system clean from carbon build-up when drilling dry or with air/oil mist, a coolant jet was added downstream of the nose piece to enable the line to be
washed.

So each material in the stack requires its own specific method for cooling and "chip" evacuation (drilling CFRP does not produce chips as they are traditionally considered in machining, but anyway).
Other articles suggest that using liquid coolants when drilling CFRP results in less tool wear and better finish, but the reason not to use liquids when drilling CFRP (or layers of it in a stack) in this specific case seems to deal with the requirement that the hole must be both clean and dry before fasteners can be installed. Plastic composites are always a little porous to some extent and drilling will slightly break up the surface, further making it more permeable. Coolant residue in a joint sealed by a fastener would elevate the risk of corrosion in the joint.

First, the caveats, I'm not in the Aerospace industry, and I'm not knowledgeable about CFRP composites. Also, this article is about CFRP/Ti, not AL, but that said, I found the following:

The broader/commercial impact of this project will be the potential to provide an environmentally-benign coolant system for CFRP-Ti drilling applications. The aerospace industry is using more CFRP-Ti in aircraft components as it is more durable and lighter than aluminum or similar metals. However, CFRP-Ti is abrasive and difficult to drill, resulting in significant heat being generated from the force and the friction of drilling, which is detrimental to the CFRP base materials. The industry currently uses petroleum oil and water for cooling, which has following problems, (1) it does not provide adequate cooling; (2) it is not environmentally friendly and must be treated as hazardous waste; (3) it requires biocides to reduce bacteria; and (4) it is not conducive to a healthy and safe worker environment.

(The above is formatted straight from the original document)
It would appear that CFRP composite drilling, using water or petroleum-based coolants: the actual coolant effects are not enough, it creates hazardous waste, requires biocides, and are not healthy or safe for workers.

Good answer, but conclusion does not make sense.  Drilling lubrication fluid is common in metals to avoid overheating.  One might focus on the potential effects of drilling fluids on CFRP.  Need for biocides may be a bingo.  Even if fluid just seeped into the matrix a little, it may compromise it enough to cause worries.

These properties are already found in usual steel cutting fluids, including microbial pollution, they are not a problem except the treatment cost. Of course if no fluid is efficient for CFRP this would be a good reason to not use them (but the metallic layer seem to require one).


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I am licensed as a Private Pilot, Airplane Multiengine Land, what do I need to do to be able to fly as PIC in a Single Engine Land aircraft?
I basically got my Multi Engine Rating however not a single engine rating even though most of my training was in ASEL. Now I am curious what I would have to do to be able to fly friends and family in a single engine instead of a twin.

Does this answer your question? If a pilot takes a MEL checkride and later a SES checkride, is the pilot now also qualified for MES/SEL?

I think the edit by user2617804 was unfortunate, because by changing the question and title, a lot of the answers don't answer the question anymore. This is also the case for the top voted answer...

@Opifex I agree, that should be rolled back because it isn't a clarification, it's a different question.

@RyanMortensen in that case I'm sorry for approving that edit. To me, it seemed like the same question. You can roll back that change by going to the revisions page and the clicking the Rollback link next to the revision you want to roll back to.

No, you cannot. Just like an ASEL doesn't give you the ability to fly an AMEL, if you have an AMEL that doesn't give you ASEL privileges.
Adding the ASEL shouldn't be that hard though, especially if you've done a majority of your training in one. You will still need to take a check ride.

To add some reasons - engine failure procedures are different if you have one or if you have more, and multi-engine craft don't generally need to worry about prop torque effects.

Since the question got changed, would you mind adding/updating your answer?

Ron, the original version of the question is answered in your post, but can you please update it as the question is a bit different now?

@Farhan I will update when I have a few minutes.

You will need to take a checkride with a DPE (or FAA inspector) to be able to be pilot-in-command in an ASEL airplane.  Since your license is AMEL only that is the only category and class or aircraft you can be pilot-in-command.
The answer is found in the Private Pilot ACS page A-12.
You hold a AMEL rating and so you follow the AMEL column for the tasks that must be completed.

I.F. - Performance and Limitations
I.G. - Operation of Systems
II.A. - Preflight Assessment
II.B. - Flight Deck Management
II.F. - Before Takeoff Check
IV.A. - Normal Takeoff and Climb
IV.B. - Normal Approach and Landing
IV.C. - Soft-Field Takeoff and Climb
IV.D. - Soft-Field Approach and Landing
IV.E. - Short-Field Takeoff and Maximum Performance Climb
IV.F. - Short-Field Approach and Landing
IV.M. - Forward Slip to a Landing
IX.A. - Emergency Descent
IX.B. - Emergency Approach and Landing (Simulated)
IX.C. - Systems and Equipment Malfunctions

These are the required items to be checked for an ASEL addon with a pilot who already holds a AMEL rating.  The examiner, at his choosing, can test on any area of the ACS during the practical test.

To answer the question, no.  You cannot fly friends and family in a SEL aircraft if your pilot certificate says MEL.
To add SEL, talk to your local FSDO about the requirements.  You can certainly get it with a checkride, but there may be another option...
It might be different with just a private certificate, but I was an ATP Multi engine only, and had single engine commercial added to my certificate years later on the basis of prior experience.  (I was also a military pilot, so that could change things.)
Literally everyone I talked to, (CFIs, Chief Instructors, a DPE...) told me I needed a single engine commercial checkride, but that turned out to be not true.
Anyhow, go to the FAA's website and fill out all your hours in IACRA then call and set up an appointment with an evaluator.  Good luck, and please correct me if I am wrong.
ADDENDUM:
it is not my intent to create false hope for the OP, and I am willing to concede this may not work, but around 2008/2009 the FAA changed their “look back” rules from 12 months to indefinite.  This is what allowed me to obtain SEL on the basis of prior experience.  Because at the time of my ATP checkride I was current in multi, and took the checkride in a twin, but hadn’t flown a single in several years.  Therefore, at that time, the examiner could only grant MEL.
My advice is don’t go in there pleading that “most” of your training was in a single.  I didn’t get SEL on the basis of “most” of my time in a single, I got it because the time I had in singles, and the maneuvers I had performed in singles, far exceeded the requirements for a commercial certificate.
Instead, go through your logbook with a fine tooth comb and be ready to make a case that your time in singles exceeded the minimums needed for a PPL, (dual, solo, night, x-country, etc.) and that later meeting the checkride standards in a multi should permit them to grant SEL on the basis of the look back rules.
If your stand-alone  SEL time falls short of the mins needed for a PPL you may not be able to make a convincing case for yourself.
Again, good luck, and I would appreciate any feedback on this approach if you attempt it.

It sounds like you qualified for single engine commercial based on prior military experience, as you suspected. The regulations on that are in 61.73 and there’s no checkride required.

@Pondlife, yes, that’s exactly what I stated in my answer. However, there are a few things to add: Section 61.73 deals with initial attainment of an FAA pilot certificate, but I already had one, obtained via checkride. And it doesn’t address  obtaining an SEL if you already have MEL. As I mentioned, it may not work for non-military private only, but the point of my answer is to ask someone capable of actually making the call and not relying on the advice of internet strangers. Because in my case, everyone was wrong...


text:
Can I become a reserve fighter plane pilot in the Indian air force?
I am a B.Tech Computer Science student in India. Soon after my studies I will get a job somewhere in computer science field. I am interested in technology, and things like Artificial Intelligence that is why I joined this technological field. But, I have huge interests in flying aircraft and also some other fields. When I was young, I wanted to be a pilot.
We have reserve army in India, so do we have reserve air force also?
If yes, how can I become reserve fighter pilot in it?
What is the procedure for it?
In this way my two other dreams will also fulfill that are flying and
serving nation in part of the year.
So as a conclusion, in major part of year I want to work in technology (AI in computer science) and in remaining part of year, I want to serve the nation as reserve fighter pilot.

While there may be members of the IAF that post here, you would be best served speaking with a recruiter.  I am a former US military pilot, but I would give the same advice to a US citizen because while some requirements are constant, others change based on current staffing levels.

I'm not sure how it works in India, but in the US this would be difficult to do. Most USAFR pilots are ex-military, in order to become a pilot you would need to be a commissioned officer and complete OCS, as well as the pilot training. As far as I can tell, India does not have a reserve air force.

@RonBeyer, Bro people asked me to ask this separately, that's why I asked here, also I did not get its answer in previous question and i also deleted my previous question because that was not well framed. So i do not think that this is duplicate.

@RonBeyer, so is there something like this reserve air force in usa?

@MichaelHall, so is there something like this reserve air force in usa?

Yes, the USA has reserve forces, but as Ron mentioned, pilots generally do a significant stint on active duty before going reserve.

And while the US does allow foreigners to join its military and earn citizenship, that requires years of active duty and the odds of getting selected as a fighter pilot is ... not good.


text:
Can a stopway withstand thrust of B737 engines on takeoff if the aircraft is lined up on the edge of the runway?
Can the thrust of Boeing 737-300 engines break the asphalt surface of a stopway marked with yellow chevrons? Can anyone give a documented reference?

Why do you think it couldn't withstand the thrust? Aircraft take off from the edge of the runway all the time.

As far as I know the yellow chevron asphalt is there precisely to withstand jetwash.


text:
Why does a radar altimeter show negative values on the ground?
This applies to the Airbus 320 or Boeing 737, among others:
Although it is useless on ground, the radar altimeter will repeatedly display a value (including negative) for a specific runway and airport on the PFD.
Why would a certain runway position be -4 radar altitude as opposed to 0?

Good question - welcome to Av.SE!

The zero reading is calibrated for main-wheel touchdown in a landing attitude. With the antenna forward of the main gear, it is, at that point, higher than it will be after the nosewheel is lowered to the runway.
So to put some rough numbers to that, in the landing attitude, mains on the ground, the antenna is roughly 8 feet above the runway, but it reads 0 -- a bias of -8 feet is applied to the display as compared to the measured value.  With all wheels on the ground, the antenna is about 4 feet above the pavement (about that on the 737, a bit more on the A320 with its taller landing gear). Now, the bias of -8 gives the display value of -4 (i.e. 4' above pavement + -8' bias = -4' displayed).
Addressing issues mentioned in comments:
Putting the antenna directly between the main gear would be difficult on a 737 because as the wheels retract inward, there isn't a lot of space between them. I'd suspect that as far as places go to mount an antenna, right there isn't a great one just because of all the dirt & hydraulic fluid & general grime in and around the wheel wells. Somewhat forward, where everything inside the aircraft (i.e. everything except the external structure of the antenna) is sealed off from all dirt & such is probably lots better for the longevity of sensitive electronics.
Also, even mounting an antenna directly in line with the gear wouldn't remove the need for a bias... it's still 4' or so off the ground when the wheels touch and you want a displayed "0".  You'd just get rid of the effect that the displayed value goes slightly negative as the nose comes down.
As I understand the system, the rad alt beam isn't all that directional... if the deck angle is +10 degrees, the measured distance is the vertical distance from the antenna to the ground - not the distance of the (hypotenuse) line 10 degrees forward of vertical.  You'd get the latter with a laser pointer aimed "straight down" from the aircraft, but the rad alt just transmits a pulse, and the first return it gets (i.e. from the nearest surface, irrespective of relative angle) is what's used to compute the height.

Those radar altimeters sound quite accurate.

Absolutely, they are. In the flare, I can read the radar altimeter digital values in the heads-up display, and as it counts down from 1 to 0, the main wheels touch.

@rclocher3 Note that radar altimeters get more accurate the closer you are to the ground. You won't get 1 ft accuracy at 2000 ft. In fact, above 2500 ft the radar altimeter value is usually not shown at all.

Do you know where the radar unit physically is on the aircraft? At first blush the need for a bias could be eliminated by mounting the antenna in line with the main gear, but of course in a landing attitude the reading will still be the slant distance (hypotenuse of a right triangle) rather than the true height.

I understand that the accuracy may be ±1% or so, with possible slant range issues as mentioned by @randomhead, but I'm impressed that one can be calibrated to be accurate to 1 foot on or near the ground, given that light travels 1 foot in about 1 ns.  Of course it travels to the ground and back, so the radar pulse would take 2 ns to bounce off ground 1 foot away and return.

@rclocher3 Wait until you see what laser distance meters used in construction can do.

@rclocher3 Not to mention laser doppler vibrometers (LDVs) which can measure changes in position of a few millionths of a meter.

@rclocher3 there are clever ways to electronically measure time-of-flight down around 10-100 picosecond resolution without even any really exotic components :)

+1, especially for addressing the sometimes silly comments.

I'd think the point of the altimeter at low altitudes is you really want to know when any part of the plane is within likely range of touching the ground, so it makes sense to include a bit of fudge room to account for a range of reasonable angles of the plane.

@DarrelHoffman  Yes - and a purely directional beam would show increasing height as the roll angle increases... not a desirable effect when approaching the flare, especially with a wing-low crosswind correction!

Sorry about the "silly" off-topic discussion of the physics of radar altimeters.  For anyone interested in how the electronics work, see the electronics.SE question "How do IR rangefinders accomplish centimeter precision without high speed / high cost parts?"

@alephzero indeed...ldv's are used to 'spy' on others by measuring the vibrations caused by speaking in a room on a window...

@randomhead The B737 radalt LRUs are mounted on the aft bulkhead of the EE bay which is between the nose gear bay and the cargo bay. The transmit and receive antennas are mounted several feet apart along the keel about half way between the nose and main gear. Since the measurement is in the LRU, it has to account for the round trip time including the antenna cable lengths in addition to the antenna height above the ground in landing configuration. This 'installation delay' is configurable via programming pins. And the antennae have to be far enough apart to avoid receiving a side lobe signal.

BTW the device in question is the Radio Altimeter. The radar altimeter is a device earlier in use to measure height above sea level/ terrain. I think it was used during pressure pattern navigation, which was used on oceanic flights. So it’s usage was at higher altitudes.


text:
Can buckling of wing skin panels occur without external compressive loads?

State whether True or False: In general, the buckling of thin and slender structures occurs under applied axial compressive stresses. In the case of wing skin panels, they can fail in buckling even if no external compressive loads are applied.

I have read that buckling occurs only in compressive loads. But the answer given is true.

You might find this interesting.

Well, two reasons for the validity of the answer, it is true indeed.

Wing bending moment. In the image above (earlier used in this answer) it shows the bending loads from lift, distributed over the wing. So the wing is loaded in bending, the bending moment is countered by the wing skin panels. Causing tension in the lower wing area, compression loads in the upper wing area, which has to be dimensioned for buckling under 2.5g loads.
So one could still maintain that the wing skin panels are under compressive loads, not the case for…..

Shear. Wings not only bend upwards, but twist as well. Due to the constant $C_M$ of asymmetrical profile wings, the wing tip wants to twist to a higher AoA, causing shear in all the wing panels. If the shear stresses exceed the dimensional/elastic buckling dimensions of the wing plate, it will deform in a wavy pattern, which is also a form of buckling.

So yes, wing plates can buckle without compressive loads, from shear stress. The good thing is that plates that are supported on all sides can still carry stress when buckled, unlike a long beam supporting a vertical load.

Depending on what they mean by "no external compressive loads", you could also see buckling of the wings as a result of a hard landing in aircraft with wing-mounted landing gear.

shear and compression also occur on fuselages.


text:
What is the capacity of the vent surge tank in an A320?
If the wing tanks of an A320 are fully filled and the fuel expands due to high temperatures then the fuel overflows into the vent tank. My question is, does the fuel get stored in the vent tank? If yes, how much? If not, does all of the fuel just spill on the ramp?

From Airbus flight crew operating manual (FCOM):

The expansion tanks above the center tank allow the center tank to be filled to the maximum. Tanks capacity is 23,858 liters.
The FCOM says the fuel can expand up to 2 % without spillage in the vent/surge tanks. That's about 239 liters per surge tank.
The volumetric expansion of the fuel is about 0.095% per °C. 2% allows for a 21°C temperature increase.
The vent/surge tank communicates with the outer tank on the same side, and fuel can flow back by gravity, when conditions permit.

Note that this is not unique to the a320 or Airbus in general.  Most large planes have similar features in this respect.  The actual numbers may vary of course, but the principle remains the same.  (Note: I'm not a pilot or aircraft mechanic, but I helped write training software for simulating exactly this system on a few models of larger military planes.)


text:
What are the minimum requirements for passing medical for PPL?
Where can I find the UK requirements to pass the medical exam for a PPL?
Basically I would like to check my health and all the requirements with any doctor - I have health insurance plan, and when I know that I meet all the criteria I go back to the aviation doctor for checking and issuing the certificate.

Related: https://www.caa.co.uk/WorkArea/DownloadAsset.aspx?id=4294973027

Have you gone to see an aviation medical examiner? You should direct this to him/her.

I don't really know what you're asking here. The UK medical standards are available online and I assume you could go to any doctor with that information for a checkup. It sounds like you aren't happy with the examination and costs so far, but I don't know what you're asking for.

@Pondlife what I was asking is the information that you just provided, for the purpose you have just mentioned  I assume you could go to any doctor with that information for a checkup. If you bother to put that in an answer I am glad to accept it, otherwise, thank you much appreciated.

You're welcome! I simplified your question to make it more focused on your main point, but you can just roll back or edit again if my changes aren't useful.

The UK aviation medical standards are available online, so I assume you could go to any doctor with that information for a checkup.